Reconstructing Fluid Expulsion and Migration North of the Variscan Orogen, Northern England

Abstract

ABSTRACT Lower Carboniferous platform carbonates in northern England host lead-zinc-fluorite-barite deposits that have previously been classified as Mississippi Valley-type (MVT). The controls on the distribution of low-temperature, particularly MVT, minerals have long been debated, but the interplay of basin evolution and fluid movement is often neglected. Early Carboniferous sedimentation on the Derbyshire Platform took place within a back-arc extensional regime, north of the northward-migrating Variscan orogenic front, which increasingly influenced fluid movement on the platform throughout the Carboniferous. It is the interplay of fluid movement, cementation, and Variscan tectonism that is the focus of this paper, which aims to reconstruct fluid movement within a basin moving from an extensional to a compressional regime. In particular, it demonstrates the variability of fluid flux during post-rift subsidence and basin inversion and the corresponding influences of this upon mineral distribution. During early burial of the Derbyshire Platform, pore-filling cements were precipitated from meteoric porewaters, driven downdip from the east under topographic drive. These cements occluded most interparticle porosity, and therefore permeability became fracture-controlled. The waning effects of extensional tectonism during post-rift subsidence permitted intermittent expulsion of small volumes of trace metal-charged, carbonate-saturated fluids from overpressured contemporaneous clastic basins adjacent to the Derbyshire Platform. Overall, however, waning tectonism meant that the volume of fluids released during this period remained minor. With the onset of the Variscan Orogeny, and resultant compressional tectonism and basin inversion, Caledonian basement fault systems were reactivated, channeling large volumes of trace metal-charged fluids from the basins onto the Derbyshire Platform, establishing an east-west component to fluid flow. The reconstruction of fluid expulsion and migration within this tectonically active regime demonstrates variations in fluid source and migration pathways during basin evolution. Consequently, fluid movement is complex, with the platform receiving fluids from a variety of sources. This highlights the importance of understanding both the temporal and the spatial variations in cement chemistry when reconstructing flow. The results of this study have relevance to other mineralized carbonate platforms in extensional and compressional settings worldwide and especially to sedimentary basins that host MVT deposits and hydrocarbons, where it is crucial to understand the timing and mechanisms of diagenetic cementation and hydrocarbon emplacement.